The development of cancer is a complex process involving the interplay of many genetic and epigenetic events. Cells undergo extensive biochemical and structural alterations throughout the course of cancer development. Tremendous efforts have been invested to increase the survival rate of cancer patients through the development of early detection programs and novel therapeutic strategies. Traditional histological standards for cancer diagnosis are well established and prognostic criteria routinely evaluated include size, invasiveness, involvement of adjacent structures or lymph nodes, metastases, and histological grade. More recently, advances in cellular and molecular techniques have fostered a better understanding of genetic and epigenetic changes during cancer development. An increasing number of molecular tumor markers have been identified to provide additional information for the diagnosis and prognosis of the disease. However, only a limited number of them have been reproducible and clinically relevant for cancer patient management. For most types of cancer, specific and sensitive markers that can predict the biological behavior of cancer cells (recurrence and metastases) are still lacking. What is needed are improved methods to specifically detect, characterize, and monitor the specific types and the progression of cancer. Furthermore, useful tumor markers that can accurately predict the outcome of patients suffering from cancers of various types and at various stages are desired.